1. Field of the Invention
This invention relates to methods and apparatus for determining a geographic position of a mobile telephone station, in combination with automated testing of cellular telephone equipment; and in particular to such methods and apparatus including elevated monitoring sites for active and passive testing of cellular base stations and mobile stations.
2. Description of the Related Art
Self-testing capabilities of cellular telephone sites currently includes test functions such as monitoring antenna performance by measuring the voltage standing wave ratio (VSWR), and monitoring site controller malfunctions and environmental conditions such as air conditioning and power failures. Generally, self-testing of electronic equipment detects relatively major malfunctions such as failures of individual radios or entire cell sites.
Currently, self-testing does not include remote monitoring of transmitted power levels to ensure that the entire signal path through the antenna is working properly and that the antenna is efficiently radiating the power delivered to it. Use of test mobiles at a site is known, but these are used to detect major failures, i.e., a problem is detected if the test mobiles do not respond when called. What is needed is an automated system which tests a complete end-to-end signal path, and which tests all the channels (frequencies, time slots, or PN codes) assigned to a particular cell site so that the entire system, including audio paths, is tested. Such a test system would not only detect equipment failures, but also detect more subtle changes in timing parameters, operational parameters, and system settings.
Such a testing system, properly designed, can also determine the location of cellular mobile stations in emergency situations. The familiar xe2x80x9c911xe2x80x9d is widely used as an emergency telephone number; 911 requests are relayed to the proper emergency-services department for response. The effectiveness of emergency services depends, of course, on emergency personnel being able to get to the caller without undue delay. To this end, the U.S. government has recently promulgated regulations requiring that providers of cellular telephone service be able to supply information on the position of a mobile station making an emergency call.
A problem with responding to requests for emergency assistance is that persons using mobile telephones often are unable to give their exact location when making an emergency call. A caller who happens to be in unfamiliar terrain does not know local landmarks, and may not know the name of the street or road on which he is driving. This lack of information regarding the caller""s location hampers and delays efforts to get emergency assistance to the caller, whether the request is for medical assistance, the police, or roadside repairs.
The geographic size and shape of each cell in a cellular telephone network is largely determined by the coverage of transmit/receive antennas located at the central cell site, and by the surrounding terrain. In open country, cells are substantially circular, with overlapping borders. Where buildings or uneven topography block radio-frequency transmissions, the cells may be quite irregular in shape.
A mobile telephone station is handed off from cell to cell based on the received signal strength and signal quality at each cell site, with the call being routed through the cell receiving or providing the best signal. In a metropolitan area, a number of cells may be capable of receiving a 911 emergency call from a particular mobile station. Triangulation, using the signal time-of-arrival at three or more sites, angle of arrival, or some combination of observable signal parameters, may be used to determine the position of such a caller. In rural areas, however, cell sites are generally too widely spaced for triangulation to be possible. This gives rise to situations in which a caller makes an emergency 911 call, is asked his location by the dispatcher, and is unable to give accurate information. Emergency services are thus delayed, sometimes with serious consequences. What is needed, therefore is a system combining automated testing functions with the ability to determine a geographic position of a mobile station.
Parameters of cellular system performance, both base stations and mobile stations, are measured at automated monitoring sites. At least some of the monitoring sites are located at high-elevation sites to give a large-radius radio horizon, and/or at existing cell sites. A microprocessor, under either local or remote control, is placed at each monitoring site. The microprocessor controls a number of xe2x80x9ctest mobiles;xe2x80x9d the test mobiles are interconnected to transmit/receive antennas. Power output of the test mobiles can be varied by the control microprocessor; the test mobiles are also capable of transmitting without the usual call set-up process, xe2x80x9ccreativelyxe2x80x9d violating their usual protocol in order to test system functions.
By slowly increasing or decreasing transmitted power on a particular frequency, the monitoring site can periodically test the power level required to acquire service on each channel served by a cell site, and the power setpoints or xe2x80x9cdynamic power controlxe2x80x9d properties the site exhibits. Hand-off performance between cells is tested by increasing power on one test mobile and decreasing it on another at a different location, and recording the levels at which the hand-off occurs. Advanced protocols such as IS-136 and IS-95 require the test mobile to manipulate its reporting of signal parameters received from a serving site, but handoff testing may be accomplished in a conceptually similar way.
Recording time-of-arrival at several monitoring locations of a signal from a mobile station making an emergency 911 call provides data for determining a position solution for the mobile station. The present invention determines TDOA (time difference of arrival) by separating a received signal into its component frequencies, and plotting phase differences for individual frequencies at different receiving sites, either cell sites or monitoring sites. A slope of the phase-shift plot is proportional to the TDOA at different sites. The arrival times thus determined are used to plot a geographic location of the mobile station generating the call.
Data processing for the positioning functioning is done by a central E-911 computer, which may also be linked to the mobile telephone switching office (MTSO) computer. Occasionally data will be available from only the serving cell site and one other site, in which case the mobile""s position can be narrowed to two possible choices. Comparison of the two locations to a map showing locations of roads, etc., can often eliminate one of the positions. Data from a combination of three sites, either cell sites or monitoring-only sites, using a time-of-arrival algorithm, will give a specific geographic location of the mobile station.
The same transmitter-locating technology may also be used for xe2x80x9cvalue-addedxe2x80x9d services such as fleet vehicle tracking, non-emergency motorist assistance, etc., limited only by the imaginations of service providers. Thus for the purposes of this application, xe2x80x9c911 callsxe2x80x9d may include non-emergency calls or mobile registration actions which also are used to generate location fixes.
Based on the above, it is an object of this invention to provide a method by which operational parameters of cellular telephone sites may be monitored on an ongoing basis.
Another object is to provide a combination of a testing and monitoring system which further uses time data and/or other observable signal parameters collected by a monitoring site and by cellular base stations to automatically provide an accurate geographic location of a mobile station placing an emergency 911 call, or to monitor location of certain mobile stations on a continuing basis, utilizing routine transmissions including registration and/or non-emergency transmissions.
A further object of the invention is to provide such a method which is automated, and which periodically tests the complete signal path or channel for each cellular frequency served by a particular site.
Another object is to provide a method by which several cellular sites may be monitored and tested by a single monitoring site located at a high elevation.
The specific nature of the invention, as well as other objects, uses, and advantages thereof, will clearly appear from the following description and from the accompanying drawings, the different views of which are not necessarily scale drawings.